1. Field of the Invention
The present invention relates to analog attenuators used in the measurement of AC signals, and more particularly to automatic compensation of the AC attenuator using a digital to capacitance converter for improved frequency response of the AC attenuator.
2. Description of the Related Art
Scientists and engineers often use instrumentation or data acquisition systems to perform a variety of functions, including test and measurement, laboratory research, process monitoring and control, data logging, analytical chemistry, test and analysis of physical phenomena, and control of mechanical or electrical machinery, to name a few examples.
A typical analog measurement instrumentation system includes signal conditioning logic which amplifies low-level signals, attenuates high level signals, and also isolates and filters signals for more accurate and safe measurements.
More specifically, a typical analog measurement system includes an AC amplifier/attenuator which either amplifies or attenuates a received analog signal depending upon the level of the signal being measured. An AC attenuator comprises two resistors that form a voltage divider to reduce the input voltage to a smaller value. The AC attenuator essentially comprises one very large resistor and one very small resistor coupled in series. The majority of the voltage appears across the larger resistor and a much smaller voltage appears across the smaller resistor wherein this voltage is measured as the output of the AC attenuator. For example, in the case of 100 to 1 AC attenuator, the large resistor is 99 times greater than the small resistor and thus 1% of the voltage appears across the smaller resistor. An example prior art voltage divider is shown in FIG. 1.
AC attenuators generally have a limited bandwidth. In other words, the frequency response of an AC attenuator is flat for a portion of the bandwidth, but is nonlinear at higher frequencies. The reason for this is that a stray or parasitic capacitance exists around the large resistor and the attenuator. At high frequencies, this parasitic capacitance forms a linkage path around the larger resistor. This parasitic capacitance also exists across the small resistor. However, its effect is much less with respect to the small resistor due to the much smaller resistance provided by the small resistor. This parasitic capacitance across each of the small and large resistors can be modeled as a capacitor coupled in parallel with each of the respective resistors. This parasitic capacitance is shown in FIG. 1 as the capacitors C1 and C2.
As the frequency of the AC signal increases, these capacitors have reduced impedance, thus essentially short circuiting the resistors in the AC attenuator. Thus, at high frequencies the small impedance offered by the parasitic capacitance affects the proper operation of the voltage divider, providing a nonlinear frequency response and thus producing inaccurate results.
As shown in FIG. 2, as the frequency of the AC signal increases, the overall impedance offered by the larger resistor becomes less and thus the attenuation factor is not as great. Thus, the output voltage starts rising, e.g., at 20 khz. The voltage continues to rise until the parasitic capacitance around the smaller resistor begins to take affect, which is generally at 100 times the frequency where the non-linearity begins for a 100 to 1 AC attenuator. At this point, the output voltage stops rising and flattens out to a new value, which essentially is the ratio between the impedance of the capacitors. At this point, the resistors are no longer significant because the capacitors offer less impedance.
FIG. 3 illustrates an AC attenuator which includes compensation according to the prior art. As shown, the AC attenuator of FIG. 3 includes a small resistor (10K ohms) and a large resistor (1 MEG ohms), thus forming a 100 to 1 attenuator. The capacitors C1 and C2 represent the parasitic capacitance that occurs at higher frequencies. In order to compensate for this parasitic capacitance, prior art systems incorporate a capacitor C3 across the smaller resistor. In the case of a 100 to 1 attenuator, the capacitor C3 would generally have a value that is 100 times greater than that of C1. The capacitor C3 operates to effectively cancel out the parasitic impedance so that the frequency or roll off of the lower portion of the attenuator occurs at the same frequency that the upper portion of the attenuator rolls off. Thus, this effectively forms two attenuators: a resistive attenuator comprising the small and large resistors that operates at low frequencies, e.g., up to 20 kilohertz, and a capacitive attenuator which operates at larger frequencies.
One problem which arises is that the resistive attenuator and the capacitive attenuator are not truly identical in their attenuation ratios, this being because they are comprised of components that have uncertain tolerances. In order to compensate for this difference and properly calibrate the resistive and capacitive attenuation components, prior art systems have added another capacitor C4, which is a variable capacitor. In this system, capacitor C3 is slightly smaller than necessary, and the variable capacitor C4 is included to add the proper amount of capacitance to compensate for the uncertain tolerances. Thus, during manufacture of the AC attenuator, the technician adjusts the capacitance of the adjustable capacitor C4 until the capacitive attenuator and the resistive attenuator have substantially identical attenuation ratios. This calibration operates by providing a known reference signal into the AC attenuator, measuring the output, and then adjusting the capacitor accordingly to achieve ideal results.
U.S. Pat. No. 4,663,586 titled "Device with Automatic Compensation of an AC Attenuator" describes an AC attenuator system with an electronic mechanism for compensating an AC attenuator. The system of U.S. Pat. No. 4,663,586 is also shown in FIG. 4. This system operates to create a scaled replica of the voltage appearing at the output of the attenuator using a multiplying amplifier and a gain component. The output of the attenuator is then driven with the scaled replica to a virtual trimmer impedance, e.g., capacitor C3, the effective value of the virtual trimmer impedance is then varied by changing the scale of the replica until the proper gain value is obtained. This gain value is then programmed for usage in the AC attenuator to provide the proper compensation.
Therefore, further improved systems and methods are desired for providing automatic compensation of an AC attenuator system.